1. Field of the Invention
The invention relates generally to a control over a shift-position changing mechanism, and, more specifically, to a technology for learning a rotation stop positional-range of an actuator, which corresponds to a shift position, based on a change in a rotation stop position of the actuator when electric power is not supplied to the actuator.
2. Description of the Related Art
There is a shift-position changing mechanism, which changes shift positions (hereinafter, sometimes referred to as “shift ranges”) of an automatic transmission in response to an operation of a shift lever performed by a driver, and which is provided with an electric motor (for example, a direct-current motor) as a power source for changing the shift positions.
With such shift-position changing mechanism, the shift lever and the shift-position changing mechanism need not be mechanically connected to each other, unlike a common changing mechanism that directly changes shift positions of an automatic transmission using an operating force applied to a shift lever by a driver. This eliminates the limitation on the layout of components in a vehicle, thereby providing greater flexibility to the vehicle design. In addition, the shift-position changing mechanism thus structured is more easily fitted to the vehicle.
An example of such mechanism is described in Japanese Patent Application Publication No. 2002-323127 (JP-A-2002-323127). A shift range changing device for an automatic transmission described in JP-A-2002-323127 determines the absolute position of an actuator and changes shift positions. The shift range changing device includes: a shift range changing mechanism that changes the shift range of the automatic transmission from among multiple shift ranges including Park; an actuator that serves as a power source for the shift range changing mechanism; control means for changing the shift range of the automatic transmission to the shift range instructed by a changing command, by driving the actuator according to the changing command issued in response to an operation externally performed; power supply means for supplying electric power to the control means; and nonvolatile rewritable storage means for storing the rotational position of the actuator and the shift range position of the automatic transmission. When electric power supply from the power supply means is once shut off and then started again, the control means reads the rotational position of the actuator and the shift range position of the automatic transmission, which are stored in the storage means when electric power supply is shut off, and sets these positions as initial values.
The shift range changing device described in JP-A-2002-323127 is able to determine the absolute position of the actuator, and change the shift ranges of the automatic transmission by driving a detent mechanism to allow the vehicle to start moving.
However, a shift-position changing mechanism usually includes a mechanism that generates an elastic force between mechanically connected components, using the deflection of the component, to maintain the rotational position of an actuator at the position corresponding to the shift position when electric power supply is shut off. Accordingly, the actuator sometimes receives a rotational force from the mechanism after electric power supply is shut off. As a result, the rotational position of the actuator is sometimes changed. In the shift range changing device described in JP-A-2002-323127, if the rotational position of the actuator changes after electric power supply is shut off, the actual rotational position of the actuator may deviates from the initial value corresponding to the shift range position stored in the storage means. This may deteriorate the accuracy of the control over the rotational position of the actuator.